As means for monitoring and maintaining optical fiber lines of an optical fiber transmission system, there is an apparatus for measuring loss distribution of the optical fiber lines for their longitudinal distance by inputting a short optical pulse(a probe pulse light) to the optical fiber lines, and observing optical intensity of its Rayleigh backscattered light in a time domain. Accordingly, increase of loss and break at local points on the optical fiber lines can be detected.
It is often seen in an urban type optical network or an access optical network that an optical transmission system demultiplexes a signal light of a wavelength .lambda.s from a single optical transmitting apparatus into numbers of portions by an optical multiplexing/demultiplexing apparatus, and transmits them to numbers of optical receiving apparatuses through respective optical transmission lines. In such cases to transmit different signals to the respective optical receiving apparatuses, a time-division multiplexing system is used.
An optical switch may be arranged to make the individual optical fiber transmission lines possible to connect a pulse tester selectively for detecting increase of loss and break on each of optical fiber lines in a branch type optical transmission system. This method, however, does not permit to measure the optical fiber lines when needed, nor continuously monitor the individual optical fiber lines.
As a solution of the above-mentioned problems, the same applicant, for example, has proposed an apparatus for detecting a fault location shown in FIG. 7. Details of the proposal are explained on the Heisei 8 nen patent application No. 205677 referring to FIG. 7.
An optical transmitting apparatus 10 outputs an optical signal of a wavelength .lambda.s for a number of optical receiving apparatuses 12-1.about.12-n. The signal for the respective optical receiving apparatuses 12-1.about.12-n is time-division-multiplexed on optical signals of the same wavelength .lambda.s except for cases to transmit the same informational contents to the respective optical receiving apparatuses 12-1.about.12-n. An optical pulse tester 14 is an apparatus to generate probe pulse lights of wavelengths .lambda.1.about..lambda.n different from the wavelength .lambda.s of the signal light, and measure their reflected lights in a time domain. The optical pulse tester 14 generally uses a tunable wavelength light source to generate a probe pulse light having a desired wavelength within wavelengths .lambda.1.about..lambda.n.
An optical combiner/divider 16 is arranged to add an output light of the optical transmitting apparatus 10 and an output light (a probe light) of the optical pulse tester 14, transmit them to a port M(multiplexed light) of an optical combiner/divider 18, and return a reflected light from the port M of the optical combiner/divider 18 to the optical pulse tester 14.
The optical combiner/divider 18 divides the light introduced into the port M into n portions, and outputs them from divided light ports #1.about.#n to optical fiber lines 20-1.about.20-n respectively. The above-mentioned optical receiving apparatuses 12-1.about.12-n are connected to the end terminals of the respective optical fiber lines 20-1.about.20-n. Reflecting elements 22-1.about.22-n, which comprise, for example, optical fiber gratings, and have a reflectivity of about 0.1.about.10% for reflecting the wavelengths .lambda.1.about..lambda.n, are arranged at marked points such as connecting points of the optical fibers on the individual optical fiber lines
When monitoring or measuring condition of the optical fiber line 20-1 is desired in such an arrangement, a probe pulse light of the wavelength .lambda.1 is obtained from the optical pulse tester 14. The probe pulse light enters the optical combiner/divider 18 through the optical combiner/divider 16. The optical combiner/divider 18 distributes the incident light of the port M to all the optical fiber lines 20-1.about.20-n. The probe pulse light of the wavelength .lambda.1 is transmitted on the optical fiber line 20-1, being reflected by the reflecting element 22-1 at the optical fiber line 20-1, and is transmitted on the optical fiber lines 20-2.about.20-n without being reflected by the reflecting elements 22-2.about.22-n at the other optical fiber lines 20-2.about.20-n.
Both of the reflected lights, reflected by the reflecting element 22-1 of the optical fiber line 20-1, and Rayleigh backscattered lights from the optical fiber line 20-1, are introduced into a divided light port #1 of the optical combiner/divider 18, and Rayleigh backscattered lights from the optical fiber lines 20-2.about.20-n are introduced into the divided light ports #2.about.#n. The optical combiner/divider 18 adds the reflected lights and the Rayleigh backscattered lights, and the combined lights are obtained from the port M to the optical combiner/divider 16. The output from the port M is divided by the combiner/divider 16, being introduced into the optical pulse tester 14. That is, the Rayleigh backscattered lights of the optical fiber lines 20-1.about.20-n and the reflected lights of the reflection element 22-1 of the optical fiber line 20-1 are introduced into the optical pulse tester 14.
The optical pulse tester 14 analyzes the intensity of the reflected lights from the optical fiber lines 20-1.about.20-n in a time domain(including to display on a monitor and/or printout). Reflected pulses from the reflecting elements 22-1.about.22-n become position standards, namely, position markers.
In the arrangement shown in FIG. 7, it is difficult to measure slight faults of the respective optical fiber lines 20-1.about.20-n precisely using the Rayleigh backward scattering, since the reflected lights of the respective optical fiber lines 20-1.about.20-n return to the optical pulse tester 14 being overlapped. However, when a break or a crack occurs, a Fresnel reflected pulse is generated from the fault location, and the reflected lights from the reflecting elements 22-1.about.22-n located backward from the break or crack vanish or become weaker than usual. Accordingly, the optical fiber line having the break and so on can be identified from the optical fiber lines 20-1.about.20-n by the wavelengths .lambda.1.about..lambda.n of the probe lights, and, moreover, a location of the break and so on can be measured based on the locations(known beforehand) of the reflecting elements 22-1.about.22-n with higher measurement precision. For example, the probe light of the wavelength .lambda.1 can detect presence and location of a break of the optical fiber line 20-1 based on the location of the reflecting element 22-1 more precisely than conventional arts, and, similarly, the probe light of the wavelength .lambda.n can detect presence and location of a break in the optical fiber line 20-n based on the location of the reflecting element 22-n more precisely than conventional arts.
In the arrangement shown in FIG. 7, however, it is difficult to measure the loss characteristic of the respective optical fiber lines 20-1.about.20-n precisely. For example, when the optical pulse tester 14 generates the probe pulse light of the wavelength .lambda.1, all the backscattered lights from the optical fiber lines 20-1.about.20-n are introduced into the optical pulse tester 14. Consequently, it becomes difficult to take a loss characteristic of the optical fiber line 20-1 alone, and yet measure it precisely.
Moreover, as the reflecting elements 22-1.about.22-n having the same reflection wavelengths .lambda.1.about..lambda.n are provided on the same optical fiber lines 20-1.about.20-n, it is difficult to detect a break occurred between the reflecting elements when distance between the adjacent reflecting elements being smaller than the distance resolution of the optical pulse tester 14.